This invention relates to the maintenance of an adequate disc height, by the prevention of subsidence, following Anterior Lumbar Interbody Fusion (ALIF), which typically uses threaded metal cages or threaded bone dowels.
Anterior Lumbar Interbody Fusion (ALIF) using threaded devices such as cages and bone dowels has been in use for over ten years. Initially, threaded cages or dowels were expected to act as a stand-alone device that would promote fusion and maintain disc height without the need for posterior surgery and instrumentation of the spine. In spite of fusion rates better than 90 percent for single level fusion and 65 percent for two-level fusion, significant subsidence has been observed on follow-up X-rays at varying times following the procedure. This subsidence, or slow insinuation of the threaded devices into the vertebral bodies, has resulted in lost disc height, which in some patients has resulted in the failure to fuse and the recurrence of often very painful symptoms.
Subsidence occurs because the threaded devices are optimally placed more posterior than anterior, where there is maximum load on the vertebral body. The threaded devices also need to be placed at opposite sides from the midline of the vertebral body. This typically results in the threaded devices being placed entirely on softer bone, which is more prone to result in subsidence. The apophyseal ring, a structure within the vertebral body, provides an area of denser, stronger bone, which would be more resistant to subsidence. This apophyseal ring, however, is found only in the very outer circumference of the vertebral body. Furthermore, the apophyseal ring is present only on the anterior and lateral aspects of the vertebral body, not on the posterior aspects, and it is typically only 1 to 2 mm thick. Most threaded devices in use today are placed inside of the apophyseal ring and fail to take advantage of its strength. ALIF and threaded cages, therefore, have been used less frequently in recent years. This has resulted in the increase of anterior-posterior fusions, or 360 degree fusions, which have actually become the “gold standard” against which other technologies are being measured for reliability and successful outcomes.
The principal disadvantage of 360 degree fusions is that the patient then needs two separate operations, either on the same day or in two separate stages. Both operations are of significant magnitude with independent, significant morbidities. There are other problems as well. Although 360 degree fusions offer almost 100 percent fusion rates, there is not 100 percent satisfaction on the part of the patients. The most frequent and important cause of patient dissatisfaction occurs because the posterior portion of the operation causes significant destabilization of the back muscles, which are essential for improved health of the patient's back. The anterior approach, especially when used with mini-open techniques, would be preferable to 360 degree fusion because the morbidity associated with it is much less. If 360 degree fusion could be avoided, there would be significant benefit to the patient in terms of reduced morbidity and faster recuperation. This, in turn, would result in earlier resumption of physical activity and return to work. In addition, there would substantial reduction in cost of treatment, since one operation would take the place of two.
No devices or surgical methods in use at the present time can overcome the various problems associated with either ALIF or 360 degree fusion. One device, described in U.S. Pat. No. 6,210,442, tries to overcome some of these problems. The device consists of a single threaded implant incorporated into a winged structure that provides lateral support. This device, however, is not designed to take advantage of the apophyseal ring and thus fails to use it as a source of stability and strength. It would be desirable if the benefits of a single, ALIF surgery could be obtained while preventing the post-operative subsidence that typically occurs after ALIF.